The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. The present disclosure invention is related in general to cable systems and, in particular, to wireline cables.
Typical wireline cable designs consist of a cable core of one or more insulated conductors (packed in an interstitial filler in the case of multiple conductors) wrapped in cabling tape followed by the application of two armor wire layers. The armor wire layers are applied counterhelically to one another in an effort to minimize torque imbalance between the layers. These armor wires provide the strength needed to raise and lower the weight of the cable and tool string and protect the cable core from impact and abrasion damage. In an effort to provide additional protection against impact and abrasion damage, larger-diameter armor wires are placed in the outer layer. Torque imbalance between the armor wire layers, however, continues to be an issue, resulting in cable stretch, cable core deformation and significant reductions in cable strength.
In pressurized wells, gas can infiltrate through gaps between the armor wires and travel along spaces existing between the inner armor wire layer and the cable core. Grease-filled pipes at the well surface typically provide a seal at the well surface. As the wireline cable passes through these pipes, pressurized gas can travel through the spaces between the inner armor wires and the cable core. When the cable then passes over and bends over a sheave, the gas may be disadvantageously released.
Typical wireline designs have approximately 98% coverage with each layer of armor wire. If the coverage is too low, the armor wires may disadvantageously move along the cable and the cable may have loose wires.
Torque for a layer of armor wire can be described in the following equation.Torque=¼T×PD×sin 2α
Where:                T=Tension along the direction of the cable                    PD=Pitch Diameter of the Armor Wires            α=Lay angle of the wires                        
Referring now to FIG. 1, since the outer armor wire layer 12 of the cable 10 carries more loads and has a larger pitch diameter, the torque generated by the outer armor wire layer 12 (indicated by an arrow 13) is generally larger than the torque generated by inner armor wire layer 14 (indicated by an arrow 15), which disadvantageously results in torque imbalance for the cable 10.
Torque imbalance in the cable 10 is disadvantageous because a cable core 16 may deform into the interstitial spaces between the inner armor wires 14, reducing the diameter of the cable 10. The cable 10 may disadvantageously have more stretch and the core 16 may be damaged. As the diameter of the cable 10 is reduced, the pitch diameter of inner armor 14 has a larger percentage reduction than the pitch diameter of outer armor 12, which may further complicate torque imbalance.
It is desirable, therefore, to provide a torque-balanced and damage resistant wireline cable.